The embodiments relate to a video control device and a video control method which change the frame rate.
One type of image display apparatuses included impulse-type display devices represented by display devices using CRTs (cathode-ray tube). An impulse-type display device strongly emits light instantaneously to display an image and causes users to see the light. Another type of image display devices includes hold-type display devices represented by active matrix-type display devices in which each pixel has a memory function. A hold-type display apparatus holds the display state of written image until image of the next frame is written.
As the active matrix-type display devices, liquid-crystal display devices including thin film transistors (TFTs) are known. In a liquid-crystal display device, an image written in pixels is held for a predetermined period of time by TFTs and capacitors provided for the respective pixels.
The liquid-crystal display apparatuses have a problem of degradation of moving images due to the slow response speed thereof as well as blurring in moving image (hereinafter, referred to as moving picture blurring) by the influence of integration in the visual system due to the hold-type display.
As examples of the solution for the problems due to the hold-type display, two methods are proposed: a method of shortening the image hold time to implement display close to the impulse-type display; and a method of multiplying the frame rate of the video signal to a higher rate by using a motion compensation circuit.
These methods are described in Kurita Taiichiro, “Fundamental Deterioration of Picture Quality for Moving Images Displayed on LCDs and Methods for Improvement”, Technical Report of IEICE, EID2000-47 (2000-09), p. 13-18.
The former method to implement display closer to the impulse-type display includes problems of reduction in luminance and flicker, and the latter method to increase the frame rate includes problems of complicated converter circuit increasing the frame rate through the motion compensation technique and increasing circuit size. However, in recent years, the latter method is increasingly employed with the progress of the motion compensation technique.
In the latter method, motion vectors are detected from frame data of inputted video signal and frame data of delayed video signal. Based on the detected motion vectors, interpolated frame data corresponding to intermediate image between the aforementioned two sets of frame data are created and are inserted therebetween. The frame rate is thus increased to two times the frame rate at which the data is inputted.
However, for creating frame data and temporarily holding the frame data during the frame-rate conversion of the created frame data, frame memory is required.
By the way, because of the demand for higher image quality in recent years, the number of gradations of video signals is increasing, and there is a demand for display devices that represent each RGB color of each pixel in more than 8-bit gradations.
In the case of LCD devices, a video signal represented by digital gradation is converted to analog voltage through a source driver and supplied to the source electrode. At this time, the solution of the digital-analog converters (DAC) used in the source drivers is usually 6 bits in personal computers and 8 bits in television receivers.
As a method of increasing apparent intermediate gray levels without increasing the resolution of DACs themselves, a FRC (frame rate control) method of rounding up and down some low bits on a frame-basis is often used.
Japanese Patent Laid-open Publication No. 5-113767 discloses a technique to increase apparent intermediate gray levels using dithering for frame data after the frame rate is doubled instead of increasing the spatial resolution of the pixels.